1. Field of the Invention
The present invention relates to an injector that injects high-pressure gaseous fuel directly into a combustion chamber of an internal combustion engine.
2. Description of Related Art
It is difficult to drastically reduce pollutant gas such as nitrogen oxides, hydrocarbons or carbon oxides included in exhaust gas of an internal combustion engine, as long as liquid petroleum is used as fuel for the internal combustion engine. It has been proposed to use gaseous fuel, such as liquid natural gas (LNG, CNG), liquid petroleum gas (LPG) or hydrogen gas, in place of liquid fuel to improve exhaust gas quality of internal combustion engines. The gaseous fuel is already used in certain engines.
However, certain problems are involved in using the gaseous fuel for engines to which fuel is directly injected by injectors. That is, the sufficient durability is not realized in the injectors because lubrication ability of the gaseous fuel is low. Further, the engines are not stably operated if gases having a low cetane number, such as hydrogen gas or pressurized natural gas, are used for diesel engines because ignitability of those gases is not high. Some proposals have been made to cope with those problems, for example, by JP-A-2003-232234 and JP-A-2006-118470.
JP-A-2003-232234 proposes to use liquid fuel (such as gasoline) and gaseous fuel (such as LPG) selectively or simultaneously according to operating conditions of an engine. In this proposal, a first system for supplying liquid fuel and a second system for supplying gaseous fuel are provided. When the fuel supplied to the engine is switched to the gaseous fuel, a liquid fuel is added to the gaseous fuel. In this manner, it is avoided that an air-fuel ratio in an intake mixture becomes too lean, which results in misfiring and bad driveability. In this system, however, the engine operating region where the liquid fuel is used is relatively large, and therefore an advantage of using the gaseous fuel is not sufficiently attained. In addition, since two systems including injectors and fuel tanks for supplying the liquid fuel and the gaseous fuel are required, an entire fuel supply system becomes large and expensive.
JP-A-2006-118470 proposes to improve lubrication in an injector by partially using the liquid fuel together with the gaseous fuel. A valve seat of the fuel injector is lubricated by the liquid fuel, and ignitability of mixture is improved by the liquid fuel injected together with the gaseous fuel injected from injection holes at a high speed. If liquid fuel having a high cetane number, such as light oil or DME (dimethyl ether) is used as the liquid fuel to be mixed with the gaseous fuel, self-ignition is possible by compression without using a spark plug. In this proposal, however, the high-pressure liquid fuel is directly supplied to a sliding portion of an injector needle. Therefore, an amount of liquid fuel injected together with the gaseous fuel tends to fluctuate, causing deviation in combustion pressures among cylinders. To reduce the fluctuation in the amount of the liquid fuel supplied, it is required to manufacture the injectors with a very high precision, resulting in a high manufacturing cost.